The background of the invention can be described most clearly, and hence the invention can be taught most effectively, by subdividing this section in three subsections. The first subsection will provide some general background regarding the role of crosslinked (and especially stiff and strong thermoset) particles in the field of the invention. The second subsection will describe the prior art that has been taught in the patent literature. The third subsection will provide additional relevant background information selected from the vast scientific literature on polymer materials science and chemistry, to further facilitate the teaching of the invention.
A. General Background
Crosslinked polymer (and especially stiff and strong thermoset) particles are used in many applications requiring high stiffness, high mechanical strength, high temperature resistance, and/or high resistance to aggressive environments. Crosslinked polymer particles can be prepared by reacting monomers or oligomers possessing three or more reactive chemical functionalities, as well as by reacting mixtures of monomers and/or oligomers at least one ingredient of which possesses three or more reactive chemical functionalities.
The intrinsic advantages of crosslinked polymer particles over polymer particles lacking a network consisting of covalent chemical bonds in such applications become especially obvious if an acceptable level of performance must be maintained for a prolonged period (such as many years, or in some applications even several decades) under the combined effects of mechanical deformation, heat, and/or severe environmental insults. For example, many high-performance thermoplastic polymers, which have excellent mechanical properties and which are hence used successfully under a variety of conditions, are unsuitable for applications where they must maintain their good mechanical properties for many years in the presence of heat and/or chemicals, because they consist of assemblies of individual polymer chains. Over time, the deformation of such assemblies of individual polymer chains at an elevated temperature can cause unacceptable amounts of creep, and furthermore solvents and/or aggressive chemicals present in the environment can gradually diffuse into them and degrade their performance severely (and in some cases even dissolve them). By contrast, the presence of a well-formed continuous network of covalent bonds restrains the molecules, thus helping retain an acceptable level of performance under severe use conditions over a much longer time period.
Oil and natural gas well construction activities, including drilling, completion and stimulation applications (such as proppants, gravel pack components, ball bearings, solid lubricants, drilling mud constituents, and/or cement additives), require the use of particulate materials, in most instances preferably of as nearly spherical a shape as possible. These (preferably substantially spherical) particles must generally be made from materials that have excellent mechanical properties. The mechanical properties of greatest interest in most such applications are stiffness (resistance to deformation) and strength under compressive loads, combined with sufficient “toughness” to avoid the brittle fracture of the particles into small pieces commonly known as “fines”. In addition, the particles must have excellent heat resistance in order to be able to withstand the combination of high compressive load and high temperature that normally becomes increasingly more severe as one drills deeper. In other words, particles that are intended for use deeper in a well must be able to withstand not only the higher overburden load resulting from the greater depth, but also the higher temperature that accompanies that higher overburden load as a result of the nature of geothermal gradients. Finally, these materials must be able to withstand the effects of the severe environmental insults (resulting from the presence of a variety of hydrocarbon and possibly solvent molecules as well as water, at simultaneously elevated temperatures and compressive loads) that the particles will encounter deep in an oil or natural gas well. The need for relatively lightweight high performance materials for use in these particulate components in applications related to the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells thus becomes obvious. Consequently, while such uses constitute only a small fraction of the applications of stiff and strong materials, they provide fertile territory for the development of new or improved materials and manufacturing processes for the fabrication of such materials.
We will focus much of the remaining discussion of the background of the invention on the use of particulate materials as proppants. One key measure of end use performance of proppants is the retention of high conductivity of liquids and gases through packings of the particles in aggressive environments under high compressive loads at elevated temperatures.
The use of stiff and strong solid proppants has a long history in the oil and natural gas industry. Throughout most of this history, particles made from polymeric materials (including crosslinked polymers) have been considered to be unsuitable for use by themselves as proppants. The reason for this prejudice is the perception that polymers are too deformable, as well as lacking in the ability to withstand the combination of elevated compressive loads, temperatures and aggressive environments that are commonly encountered in oil and natural gas wells. Consequently, work on proppant material development has focused mainly on sands, on ceramics, and on sands and ceramics coated by crosslinked polymers to improve some aspects of their performance. This situation has prevailed despite the fact that most polymers have densities that are much closer to that of water so that in particulate form they can be transported much more readily into a fracture by low-density fracturing or carrier fluids such as unviscosified water.
Nonetheless, the obvious practical advantages [see a review by Edgeman (2004)] of developing the ability to use lightweight particles that possess almost neutral buoyancy relative to water have stimulated a considerable amount of work over the years. However, as will be seen from the review of the prior art provided below, progress in this field of invention has been very slow as a result of the many technical challenges that exist to the successful development of cost-effective lightweight particles that possess sufficient stiffness, strength and heat resistance.
B. Prior Art
The prior art can be described most clearly, and hence the invention can be placed in the proper context most effectively, by subdividing this section into two subsections. The first subsection will describe prior art related to the development of “as-polymerized” thermoset polymer particles. The second subsection will describe prior art related to the development of thermoset polymer particles that are subjected to post-polymerization heat treatment.
1. “As-Polymerized” Thermoset Polymer Particles
As discussed above, particles made from polymeric materials have historically been considered to be unsuitable for use by themselves as proppants. Consequently, their past uses in proppant materials have focused mainly on their placement as coatings on sands and ceramics, in order to improve some aspects of the performance of the sand and ceramic proppants.
Significant progress was made in the use of crosslinked polymeric particles themselves as constituents of proppant formulations in prior art taught by Rickards, et al. (U.S. Pat. Nos. 6,059,034; 6,330,916). However, these inventors still did not consider or describe the polymeric particles as proppants. Their invention only related to the use of the polymer particles in blends with particles of more conventional proppants such as sands or ceramics. They taught that the sand or ceramic particles are the proppant particles, and that the “deformable particulate material” consisting of polymer particles mainly serves to improve the fracture conductivity, reduce the generation of fines and/or reduce proppant flowback relative to the unblended sand or ceramic proppants. Thus while their invention differs significantly from the prior art in the sense that the polymer is used in particulate form rather than being used as a coating, it shares with the prior art the limitation that the polymer still serves merely as a modifier improving the performance of a sand or ceramic proppant rather than being considered for use as a proppant in its own right.
Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress towards the development of lightweight proppants consisting of high-strength crosslinked polymeric particles for use in hydraulic fracturing applications. However, embodiments of this prior art, based on the use of styrene-divinylbenzene (S-DVB) copolymer beads manufactured by using conventional fabrication technology and purchased from a commercial supplier, failed to provide an acceptable balance of performance and price. They cost far more than the test standard (Jordan sand) while being outperformed by Jordan sand in terms of the liquid conductivity and liquid permeability characteristics of their packings measured according to the industry-standard API RP 61 testing procedure. [This procedure is described by the American Petroleum Institute in its publication titled “Recommended Practices for Evaluating Short Term Proppant Pack Conductivity” (first edition, Oct. 1, 1989).] The need to use a very large amount of an expensive crosslinker (50 to 80% by weight of DVB) in order to obtain reasonable performance (not too inferior to that of Jordan Sand) was a key factor in the higher cost that accompanied the lower performance.
The most advanced prior art in stiff and strong crosslinked “as-polymerized” polymer particle technologies for use in applications in oil and natural gas drilling was developed by Albright (U.S. Pat. No. 6,248,838) who taught the concept of a “rigid chain entanglement crosslinked polymer”. In summary, the reactive formulation and the processing conditions were modified to achieve “rapid rate polymerization”. While not improving the extent of covalent crosslinking relative to conventional isothermal polymerization, rapid rate polymerization results in the “trapping” of an unusually large number of physical entanglements in the polymer. These additional entanglements can result in a major improvement of many properties. For example, the liquid conductivities of packings of S-DVB copolymer beads with wDVB=0.2 synthesized via rapid rate polymerization are comparable to those that were found by Bienvenu (U.S. Pat. No. 5,531,274) for packings of conventionally produced S-DVB beads at the much higher DVB level of wDVB=0.5. Albright (U.S. Pat. No. 6,248,838) thus provided the key technical breakthrough that enabled the development of the first generation of crosslinked polymer beads possessing sufficiently attractive combinations of performance and price characteristics to result in their commercial use in their own right as solid polymeric proppants.
2. Heat-Treated Thermoset Polymer Particles
There is no prior art that relates to the development of heat-treated thermoset polymer particles that have not been reinforced by rigid fillers or by nanofillers for use in oil and natural gas well construction applications. One needs to look into another field of technology to find prior art of some relevance related to such “unfilled” heat-treated thermoset polymer particles. Nishimori, et. al. (JP1992-22230) focused on the development of particles for use in liquid crystal display panels. They taught the use of post-polymerization heat treatment to increase the compressive elastic modulus of S-DVB particles at room temperature. They only claimed compositions polymerized from reactive monomer mixtures containing 20% or more by weight of DVB or other crosslinkable monomer(s) prior to the heat treatment. They stated explicitly that improvements obtained with lower weight fractions of the crosslinkable monomer(s) were insufficient and that hence such compositions were excluded from the scope of their patent.
C. Scientific Literature
The development of thermoset polymers requires the consideration of a vast and multidisciplinary range of polymer materials science and chemistry challenges. It is essential to convey these challenges in the context of the fundamental scientific literature.
Bicerano (2002) provides a broad overview of polymer materials science that can be used as a general reference for most aspects of the following discussion. Additional references will also be provided below, to other publications which treat specific issues in greater detail than what could be accommodated in Bicerano (2002).
1. Selected Fundamental Aspects of the Curing of Crosslinked Polymers
It is essential, first, to review some fundamental aspects of the curing of crosslinked polymers, which are applicable to such polymers regardless of their form (particulate, coating, or bulk).
The properties of crosslinked polymers prepared by standard manufacturing processes are often limited by the fact that such processes typically result in incomplete curing. For example, in an isothermal polymerization process, as the glass transition temperature (Tg) of the growing polymer network increases, it may reach the polymerization temperature while the reaction is still in progress. If this happens, then the molecular motions slow down significantly so that further curing also slows down significantly. Incomplete curing yields a polymer network that is less densely crosslinked than the theoretical limit expected from the functionalities and relative amounts of the starting reactants. For example, a mixture of monomers might contain 80% DVB by weight as a crosslinker but the final extent of crosslinking that is attained may not be much greater than what was attained with a much smaller percentage of DVB. This situation results in lower stiffness, lower strength, lower heat resistance, and lower environmental resistance than the thermoset is capable of manifesting when it is fully cured and thus maximally crosslinked.
When the results of the first scan and the second scan of S-DVB beads containing various weight fractions of DVB (wDVB), obtained by Differential Scanning calorimetry (DSC), as reported by Bicerano, et al. (1996) (see FIG. 1) are compared, it becomes clear that the low performance and high cost of the “as purchased” S-DVB beads utilized by Bienvenu (U.S. Pat. No. 5,531,274) are related to incomplete curing. This incomplete curing results in the ineffective utilization of DVB as a crosslinker and thus in the incomplete development of the crosslinked network. In summary, Bicerano, et al. (1996), showed that the Tg of typical “as-polymerized” S-DVB copolymers, as measured by the first DSC scan, increased only slowly with increasing wDVB, and furthermore that the rate of further increase of Tg slowed down drastically for wDVB>0.08. By contrast, in the second DSC scan (performed on S-DVB specimens whose curing had been driven much closer to completion as a result of the temperature ramp that had been applied during the first scan), Tg grew much more rapidly with wDVB over the entire range of up to wDVB=0.2458 that was studied. The more extensively cured samples resulting from the thermal history imposed by the first DSC scan can, thus, be considered to provide much closer approximations to the ideal theoretical limit of a “fully cured” polymer network.
2. Effects of Heat Treatment on Key Properties of Thermoset Polymers
a. Maximum Possible Use Temperature
As was illustrated by Bicerano, et al. (1996) for S-DVB copolymers with wDVB of up to 0.2458, enhancing the state of cure of a thermoset polymer network can increase Tg very significantly relative to the Tg of the “as-polymerized” material. In practice, the heat distortion temperature (HDT) is used most often as a practical indicator of the softening temperature of a polymer under load. As was shown by Takemori (1979), a systematic understanding of the HDT is possible through its direct correlation with the temperature dependences of the tensile (or equivalently, compressive) and shear elastic moduli. For amorphous polymers, the precipitous decrease of these elastic moduli as Tg is approached from below renders the HDT well-defined, reproducible, and predictable. HDT is thus closely related to (and usually slightly lower than) Tg for amorphous polymers, so that it can be increased significantly by increasing Ts significantly.
The HDT decreases gradually with increasing magnitude of the load used in its measurement. For example, for general-purpose polystyrene (which has Tg=100° C.), HDT=95° C. under a load of 0.46 MPa and HDT=85° C. under a load of 1.82 MPa are typical values. However, the compressive loads deep in an oil well or natural gas well are normally far higher than the standard loads (0.46 MPa and 1.82 MPa) used in measuring the HDT. Consequently, amorphous thermoset polymer particles can be expected to begin to deform significantly at a lower temperature than the HDT of the polymer measured under the standard high load of 1.82 MPa. This deformation will cause a decrease in the conductivities of liquids and gases through the propped fracture, and hence in the loss of effectiveness as a proppant, at a somewhat lower temperature than the HDT value of the polymer measured under the standard load of 1.82 MPa.
b. Mechanical Properties
As was discussed earlier, Nishimori, et. al. (JP1992-22230) used heat treatment to increase the compressive elastic modulus of their S-DVB particles (intended for use in liquid crystal display panels) significantly at room temperature (and hence far below Tg). Deformability under a compressive load is inversely proportional to the compressive elastic modulus. It is, therefore, important to consider whether one may also anticipate major benefits from heat treatment in terms of the reduction of the deformability of thermoset polymer particles intended for oil and natural gas drilling applications, when these particles are used in subterranean environments where the temperature is far below the Tg of the particles. As explained below, the enhancement of curing via post-polymerization heat treatment is generally expected to have a smaller effect on the compressive elastic modulus (and hence on the proppant performance) of thermoset polymer particles when used in oil and natural gas drilling applications at temperatures far below their Tg.
Nishimori, et. al. (JP1992-22230) used very large amounts of DVB (wDVB>>0.2). By contrast, in general, much smaller amounts of DVB (wDVB≦0.2) must be used for economic reasons in the “lower value” oil and natural gas drilling applications. The elastic moduli of a polymer at temperatures far below Tg are determined primarily by deformations that are of a rather local nature and hence on a short length scale. Some enhancement of the crosslink density via further curing (when the network junctions created by the crosslinks are far away from each other to begin with) will hence not normally have nearly as large an effect on the elastic moduli as when the network junctions are very close to each other to begin with and then are brought even closer by the enhancement of curing via heat treatment. Consequently, while the compressive elastic modulus can be expected to increase significantly upon heat treatment when wDVB is very large, any such effect will normally be less pronounced at low values of wDVB. In summary, it can thus generally be expected that the enhancement of the compressive elastic modulus at temperatures far below Tg will probably be small for the types of formulations that are most likely to be used in the synthesis of thermoset polymer particles for oil and natural gas drilling applications.